API 650 Minimum shell thickness paragraph 5.6.1.1, Note 4

[piaengr]

Does anyone know if there are any specific reasons why API has added note 4 to paragraph 5.6.1.1 in the 11th Edition of API 650, June 2007? The note states that tanks with diameters less than 50' but greater than 10.5' require a minimum thickness of 0.25" for the lowest shell course, rather than a minimum of 0.1875" from the table for the remaining shell courses. In previous versions, the minimum thickness was 0.1875" for all courses in tanks up to 50' in diameter. My understanding is that API has established these minimum thicknesses based on erection requirements. But I am curious why now they have added this new requirement. If anyone has any ideas or knowledge to share, it would be greatly appreciated.

 

[JoeTank]

I believe that API adopted this from the PIP standard. Owners of smallish tanks were also finding that external corrosion near the base of a 3/16" thick shell plate had very little margin before repairs were required. In their view, I'm sure this is money well spent.

Joe Tank

 

[piaengr]

Thank you JoeTank for the input.